1. Field of the Invention
This invention relates to a method and apparatus for measuring the shape of solid objects that have a high surface glossiness or a mirror surface finish, using an apparatus that has imaging capabilities such as an ITV camera.
2. Description of the Prior Art
In various fields of industry the shape measurement of solid objects is an essential technique for product inspection, process automation and the like. Such products include various product items such as motorcycle mufflers, machined shafts and the like that have a high surface glossiness or a mirror surface finish (hereinafter referred to simply as glossy surface).
Conventional methods of measuring solid shapes are difficult to apply to the measurement of objects such as these, that is, objects that have a glossy surface (hereinafter referred to simply as glossy objects), as conventional methods are directed at the measurement of objects having an irregular reflecting surface.
Examples of representative conventional methods of measuring the shape of solid objects include the stereo camera method in which two cameras are used to obtain an image of the same object and points common to both images are then obtained to derive the three-dimensional shape of the object, and the light section method in which a slit light is projected onto the object and a camera is used to record the locations of the slit light on the surface of the object and thereby obtain the three-dimensional shape.
However, applying these techniques to glossy objects gives rise to the following type of problem. In the case of the stereo camera method, for example, because of the surface glossiness of the object, the images that are obtained include images of the surroundings, and as a result the two images have a different density distribution, making it difficult to detect the common points. In the case of the light section method, because the glossy surface produces a regular reflection of the slit light projected onto the object, in some positions the camera may not be able to obtain an image. This is illustrated in FIG. 5.
FIG. 5 shows a shape measurement system that employs the light section method. In FIG. 5, reference numeral 10 denotes the object of the measurement, 11 is a glossy surface of the object 10, 12 is a slit light projector that can be controlled to project slit light at any desired angle, and 13 is an ITV camera for recording the position of the slit light on the surface of the object 10. The slit light projector 12 and ITV camera 13 are both arranged on axis X.sub.3, the optical axes of the slit light projector 12 and ITV camera 13 coincide with axes and Y.sub.3 and Y'.sub.3 which are perpendicular to axis X.sub.3. The distance between axis Y.sub.3 and axis Y'.sub.3 is B.sub.3, and the origin is 0.
In this arrangement, the slit light 1.sub.31 projected from the slit light projector 12 onto the measurement object 10 at an angle of a.sub.31 to axis X.sub.3 is regularly reflected from point P.sub.31 on the glossy surface 11 to become slit light 1'.sub.31. The slit light 1'.sub.31 impinges on the ITV camera 13 and focusses into an image at a point p.sub.31 on the image formation plane. As the surface of the measurement object 10 is a glossy surface 11, in accordance with the laws of reflection the angle of incidence b.sub.31 and reflex angle b'.sub.31 relative to the normal N.sub.31 of the tangent plane (not shown) at point P.sub.31 will be equal. Therefore the position of point P.sub.31 on glossy surface 11 can be calculated by triangulation, using the slit light angle of projection a.sub.31, distance B.sub.3, the slit light imaging position p.sub.31 and the focal length of the ITV camera 13.
Next, if angle a.sub.32 is the angle of slit light projection relative to axis X.sub.3, if slit light 1.sub.32 is projected at point P.sub.32 on the glossy surface 11, with the relationship between the angle of incidence b.sub.32 and reflex angle b'.sub.32 relative to the normal N.sub.32 of the tangent plane at point P.sub.32 being b.sub.32 =b'.sub.32, it will be regularly reflected to become slit light 1'.sub.32. In this state slit light 1'.sub.32 will not impinge on the lens aperture plane of the ITV camera 13, meaning that it is impossible to observe point P.sub.32 with the ITV camera 13.
Thus, when the conventional light section method is applied to the measurement object 10, scanning slit light over the whole surface of the measurement object 10, only those parts of the glossy surface 11 that satisfy the condition angle of slit light incidence=reflex angle (one point in the case of a convex object), and it is not possible to measure the whole shape of the measurement object 10. One way of obtaining the full three-dimensional shape of the measurement object 10 with the above shape measurement method is to move the above shape measurement apparatus relative to the measurement object 10, and measure by scanning with the slit light at each move. However, in order to measure one point with such a method it is necessary to scan the whole surface of the measurement object 10 with slit light. Also, when a scanning type image capture device is used such as the ITV camera 13 used in the example of a conventional apparatus, each slit light scanning angle would require measurement time for one frame obtained by the ITV camera 13. Thus, there has been the problem that the measurement time is increased.
Other shape measurement methods have been tried which attempt to directly measure the glossy surface of an object. The light projection method is one example. The light projection method consists of backlighting a rotating object and establishing the three-dimensional shape of the object from the outline formed by the shadow thus produced. However, the only information provided by this method is the outline of the object, and as such it is impossible, in principle, to measure the internal shape of the outline.
JP-B-61-17281 discloses a method of using circularly polarized light to detect the orientation of a glossy surface. This method consists of setting up numerous circularly polarized light sources around the glossy surface and using an image capture device equipped with polarized light detection means to measure light thus reflected from the glossy surface and thereby establish the shape of the object. Drawbacks of this method are that it requires a large number of circularly polarized light sources and complex equipment such as an image capture device equipped with polarized light detection means, and also involves the need to carry out complex calculations to find the orientation of a glossy surface.
Thus, while various methods have been tried aimed at measuring the shape of glossy objects, with each of the methods having drawbacks, as yet no effective means has been developed which can accomplish such shape measurement.